PABX No. 3

The GPO were not allowed to install any large PABX installation, so this was left to approved PABX suppliers.  The GPO/BT however, had to maintain the equipment once installed and connected to the Network.

PABX 3 systems were installed by Ericsson/Plessey, GEC, TMC & STC.

C 1130
Issue 2, 18.1.68

Private Automatic Branch Exchange No. 3
(P.A.B.X. No. 3)


1. Introduction
The P.A.B.X. 3 is designed to cater for P.A.B.X. installations where the required ultimate capacity will exceed 49 automatic extensions. The equipment is manufactured and installed by any of the approved manufacturers, in accordance with the arrangements described in C 3001.

2. Contents

Numbering scheme5
Standard facilities at main installations7
Standard facilities at satellite installations8
Optional standard facilities9
Non-standard facilities10
Automatic extension line circuits and line-finders SA 815111
1st group selectors SA 8152 (2000-type) and SA 8211 (4000-type)12
Final selectors SA 8153 and SA 8169 (2000-type)-SA 8213 and SA 8214 (4000-type)13
Final selectors SA 8172 and SA 8185 (2000-type)-SA 8225 and SA 8226 (4000-type)14
Lamp lighting circuit SA 815715
0-level circuit SA 816816
Exchange line circuit SA 815817
Exchange line adapter circuit SA 815418
Exchange line adapter circuit SA 815519
Enquiry circuits SA 8159 and SA 817120
2nd group, incoming or enquiry selector SA 8160 (2000-type) and SA 8212 (4000-type) 21
Inter-P.B.X. line circuit and private circuit relay-sets22
Manual extension circuit SA 816423
Long extension auxiliary circuit SA 810424
2nd group selector with one and two digit selection SA 8254 (2000-type) and SA 8239 (4000-type)25
Satellite alarm extension circuits26
Satellite night service switching circuit SA818627
Emergency circuit SA 821928
Satellite register circuit SA 824529
Satellite finder circuit SA 824430
Congestion circuit SA 824631
Auxiliary selector level access and dial tone circuit SA 828832
Ringing machine34
Vibrator ringing35
Linefinder start distribution36
Line-and-final selector rack39
Group selector rack40
Relay-set rack41
Automatic equipment rack42
Composite rack43
Ringer rack44
Satellite register rack45
Equipment rack46
Face equipment48
Position equipment49
Conference circuits SA 8216 and SA 821750
Metering circuits SA 8227 and SA 822851
Cable-turning section equipment52
Angle sections53
Supervisor's desk55
Enquiry desk56
'Direct exchange' night service59
'Subsidiary night service switchboard'60

3. General
The automatic equipment, comprising the basic units, linefinders, group selectors and final selectors, follows public exchange practice, as far as applicable, and is mounted on open-type racks. 2000-type selectors are now standard. 4000-type selectors have been provided at certain installations and may continue to be used until stocks are exhausted. The circuits, components and facilities are standard, but the equipment arrangements may vary to a small extent between manufacturers and may also be adjusted, if necessary, to meet the needs of a particular installation. The manual switchboard, which is of the cord type with an extensive multiple, in the general case consists of Sections, Switch, P.B.X., SA 7560 arranged for 50 volt working. Larger switchboards are used when necessary to provide additional multiple space.

Two position switchboard with cable turning section to the left.
The alarm indicator can be seen to the top of the cable turning section

Satellite working may be employed when it is desired to achieve economy in line plant by providing separate automatic equipments at a number of locations, while concentrating manual board services at a central point. As the provision of satellite P.A.B.X.s may involve the use of inter-P.B.X. circuits of a type not included in the overall national transmission plan individual approval of each circuit is required (C 3003 refers).


4. Trunking
A typical trunking scheme for a main installation is shown in Fig. 1. The two satellite schemes which are used, i.e. register control and non-register control, are illustrated by Figs. 2 and 3 respectively. A linked-numbering scheme is provided by the arrangement shown in Fig. 2. Registers are employed to discriminate between the various types of call originated by satellite extensions and to absorb or repeat digits as required. The trunking arrangements possible are limited by the following factors:-

(a) The first digit received must determine the number of digits to be pulsed out by the register.

(b) Second digit discrimination on extension-to-extension calls is only possible after one selected first digit.

The scheme shown in Fig. 3 employs step-by-step control and routing digits are normally required on calls between main and satellite P.A.B.X.s in both directions. It is possible to provide linked-numbering by using a separate junction route for each outgoing selector level and terminating the junctions on incoming second selectors. This arrangement, however, is expensive in lines and equipment and has not been widely used.

The register controlled system is now regarded as standard and will be used for the majority of new installations. Exceptionally, the non-register scheme may still be used where it provides the most economic way of meeting the subscriber's requirements.

More detailed trunking information, including reference to the appropriate circuit diagram numbers, is given on Diagram SA 8150. Calculation of equipment quantities is explained in C 3025.

5. Numbering scheme
A three-digit, mixed three-digit and four-digits, or four-digit numbering scheme is used dependent upon the ultimate capacity required.

(a) Main P.A.B.X. - The 1st group selector levels are normally allocated as follows:-

Level 1 - Spare [the use of level 1 in exceptional circumstances is subject to the approval of the Telecomms HQ (M2.1.1.)].

Levels 2 - 7 Extension numbers including inter-P.B.X. lines with final selector access and satellite installations (via second selectors when required).

It is usual to connect extensions to the lower levels.

Level 8 - Inter-P.B.X. circuits (either direct, or via 2nd group and/or two digit group selectors for additional groups where economical). Additional groups of inter-P.B.X. lines may be connected to levels 2-7.

Level 9 - Direct access to public exchange.

Level 0 - Manual switchboard assistance.

Manual extensions outside the automatic numbering scheme will be prefixed by the digit 0, i.e., 00 to 019 etc.

For satellite working where a linked-numbering scheme exists a 1st group selector level is required for each satellite with more than 100 extensions and a first or second selector level for each satellite with 100 or fewer extensions depending upon whether four- or three-digit numbering is employed.

When a satellite P.A.B.X. is not part of a linked-numbering scheme a first or second group selector level may be allocated.

(b) Satellite P.A.B.X. (non-register type) - The 1st group selector levels are normally allocated as follows:-

Level 1 - Spare.

Levels 2-6 - Extension numbers or inter-P.B.X. circuits etc.

Level 7 - Unidirectional or bothway circuits to the main P.A.B.X. (auto. level).

Level 8 - Spare.

Level 9 - Direct access to public exchange (if required).

Level 0 - Main manual switchboard assistance.

(c) Satellite P.A.B.X. (register type) - Initial digits are normally allocated as follows:-

Digit 1 - Spare.

Digits 2-8 - Extension and inter-P.B.X. circuits within the linked-numbering scheme.

Digit 9 - Direct access to public exchange (if required).

Digit 0 - Main manual switchboard assistance.


6. General
P.A.B.X. 3 facilities are classified as standard, optional standard (with or without an additional rental) and non-standard. These terms are defined and the procedure applicable to the different classes explained in C 3001.

7. Standard facilities at main installations
The following standard facilities are provided:-

(a) Automatic connexion of extension-to-extension calls.

(b) Operator assistance by dialling 0 (lamp per line or 0 level working).

(c) Incoming or outgoing extension-to-exchange calling via the switchboard.

(d) Extension-to-exchange direct access by dialling 9.

(e) Level 9 barring individual to extensions
This facility is controlled by means of a wiring strap in the line circuit of automatic extensions.

(f) Automatic and/or manual inter-P.B.X. connexions
Terminating relay-sets are available for the following signalling methods:-

  1. Bothway dialling, P.A.B.X. - P.A.B.X. (loop disconnect), with joint access from auto. and manual switchboard.

  2. Bothway dialling, P.A.B.X. - P.A.B.X. or manual-out with dialling-in P.A.B.X. - P.M.B.X. (S.C.D.C.), with joint access from auto. and manual switchboard.

  3. Manual-out with dialling-in, P.A.B.X. - P.M.B.X. (earth calling), with joint access from auto. and manual switchboard.

  4. Bothway manual-to-manual, P.A.B.X. - P.A.B.X. or P.M.B.X. (earth calling), with joint access from automatic and manual switchboard.

  5. Bothway manual-to-manual, P.A.B.X. - P.A.B.X. or P.M.B.X. (automatic balanced battery), with joint access from auto. and manual switchboard.

  6. Bothway manual-to-manual, P.A.B.X. - P.A.B.X. 3 or 4 (automatic generator signalling) with joint access from auto. and manual switchboard.

  7. Bothway dialling, P.A.B.X. - P.A.B.X. or manual-out with dialling-in P.A.B.X. - P.M.B.X. (S.S.A.C. 13), with joint access from auto. and manual switchboard.

  8. Bothway manual-to-manual, P.A.B.X. - P.A.B.X. or P.M.B.X. (generator or balanced battery), manual switchboard access only.

All automatic signalling terminations, i.e. (i)-(vii) inclusive, provide for through supervision on tandem connected calls. Arrangements are included in each termination to prohibit the connexion of exchange calls when used on circuits which are classified as inter-switchboard private circuits, and any one type of termination is capable of interconnexion with a similar or any other type.

(g) Hold for enquiry
By depression of the telephone button, an extension user is able to hold a public exchange call and make an enquiry call to another extension or over an automatic signalling inter-P.B.X. line. The extension user is reconnected to the exchange line by a further depression of the button.

(h) Operator call-in
The operator may be called into all exchange calls and into inter-P.B.X. calls set up via the manual switchboard by operation of the telephone button. On exchange calls a double depression is necessary (the first being for enquiry) but for other types of call a single depression is sufficient. Call-in is signalled to the switchboard in the following ways:-

(i) On calls not held on the switchboard the signal is given on the appropriate calling lamp.

(ii) On calls held on the switchboard the signal is given by flashing the supervisory lamp of the cord circuit concerned.

(j) Standard ringing and tones.

(k) Through clearing controlled by the extension telephone on all extension-to-exchange, and extension-to-automatic signalling inter-P.B.X. extensions.

(l) Trunk offering
A branching extension multiple is used and an operator can gain access to any engaged extension by over-plugging. Special trunk offering cords are provided for this purpose as the use of a normal cord circuit would interfere with supervisory signals on established calls connected via a manual position.

(m) An audible alarm with a cut-off key is provided in the switchroom.

(n) Night service switching is accomplished by the operation of the night service key. Under night service conditions all audible alarms are suppressed and the locking circuits of any generator-type calling signals are made ineffective.

(o) Extended alarm panel
This panel is located in the cable-turning section and indicates alarms on the equipment and mains power failures. Alarm cut-off facilities in the form of a 'receiving attention' key is incorporated. In addition, a separate alarm indication is provided for each satellite installation.

(p) Equipment alarms
The following are provided:

(i) Urgent alarms (e.g. fuse, release alarm) are signalled on the equipment rack concerned and extended to the manual switchroom.

(ii) Non-urgent alarms (e.g. P.G. extensions) are visually displayed in the apparatus room and extended to the manual switchroom.

(iii) At installations with 1200 extensions, sub-section and floor alarms required.

(q) First party release is provided extension calls.

(r) P.G. lock-out
If a 1st group selector is seized by an extension line circuit but not stepped within 30-60 seconds the selector is force released. The line circuit continues to be held and returns spare number tone to incoming callers.

(s) P.G. extension test facilities
At installations where 'lamp per line' working is employed [see par. 48(c)(i)] facilities are provided which enable the manual switchboard operators to identify the number of any automatic extension line which is P.G. A P.G. pilot lamp is mounted in the cable-turning section to indicate when a P.G. exists. Operation of the P.G. test key indicates the number of the extension P.G. by means of a flashing signal on the line lamp of the extension affected.

8. Standard facilities at satellite installations
The following facilities are provided:-

(a) Automatic connexion of extension-to-extension calls.

(b) Operator assistance by dialling 0
An outgoing junction to the main P.A.B.X. is seized and a signal applied to the line causes a lamp associated with the junction termination to glow. The operator answers by plugging into the appropriate answering jack.

(c) Connexion of incoming and outgoing exchange calls via tile main switchboard.

(d) Extension-to-exchange access by dialling 9
This facility may be barred to individual extensions by means of a strap in the extension line circuit. The enquiry facility is not available on calls set up in this way.

(c) Automatic and/or manual connexions to inter-P.B.X. lines via the main P.A.B.X.

(f) Automatic connexion to inter-P.A.B.X. lines terminated on the satellite (not available at register controlled installations).

(g) Operator call-in on all calls set up via the main manual switchboard.

(h) Standard ringing and tones.

(j) Calling party release on calls to and from a satellite extension. [Calling party release is not provided on satellite final selectors in view of the 're-ring' facility described in sub-par. (k) below].

(k) Trunk offering
The P.A.B.X. operator dials the required extension and on receipt of engaged tone rings on the satellite junction circuit. This disconnects engaged tone and enables the operator to speak to the engaged extension. If the receiver of the called extension is now replaced the extension is automatically re-rung.

(l) Equipment alarms
The following equipment alarms are provided :-

  • Fuse alarm.

  • Selector release alarm.

  • Ring fail (installations with machine ringing).

  • P.G. alarm.

(m) Alarm extension
Urgent alarms [(i)-(iii) above and mains fail conditions] are signalled to the main P.A.B.X. over an allocated junction. The junction is available for normal traffic except for the short interval while an alarm or an alarm cleared signal is being given.

(n) P.G. lock-out
As described in par. 7 (r).

9. Optional standard facilities
The more common optional standard facilities are described below. More detailed information is given in Specn. S 945. In certain instances standard diagrams have not been produced and the facilities are provided to diagrams in the contractors' series.

(a) Direct exchange night service
A selected extension line is directly connected to a public exchange line for the purpose of making and receiving public exchange calls over that exchange line. With this form of night service the extension loses P.A.B.X. facilities. At satellite installations the extensions are connected to a main-satellite junction by use of the satellite night switching circuit. The junctions are connected to the appropriate exchange line by the P.A.B.X. operator.

(b) Satellite night service switching
Switching may be controlled from the main switchboard by dialling a selected number in the extension range. Where this method is not adopted ringing machines are switched from continuous to start-stop working as required by a time switch.

(c) Manual extensions
Two types of manual extension facility may be provided:-

(i) Manual O/G and auto I/C. - Extensions of this type are within the automatic extension numbering range and up to five may be included in each 50 line extension group at main installations.

(ii) Manual O/G and manual I/C. - Extensions of this type are not included in the automatic extension numbering range.

(d) Ancillary working
At installations where more than two manual sections are provided, more than one lamp appearance is permitted on exchange lines, inter-P.B.X. lines and manual extensions.

(e) Grout hunting on extensions
Final selectors which give hunting facilities over a group of 2/10 extension lines are provided at main and satellite P.A.B.X.s when required.

(f) Free-line signalling
Free-line signalling is available on exchange line circuits and inter-P.B.X. line circuits.

(g) Trunk barring
Equipment may be associated with the bothway or outgoing exchange lines to prevent selected extensions with the direct access facility from obtaining trunk calls either by dialling the national number or via the public exchange 'trunk' operator. The equipment is located at the P.A.B.X, for privately purchased installations and either at the P.A.B.X. or in the public exchange, at the discretion of the T.M. (Tp.S.I. H24 VIII refers), for P.O. installations.

Trunk barring may be effected in the following ways:-

  1. Director and non-director exchanges (general case) - Barring public exchange codes commencing with the digit(s) 0 and 10. The equipment is capable of either barring both codes, or barring 10 and permitting 0.

  2. Non-director exchanges on the fringe of director areas - Barring public exchange codes commencing with the digits 10 and 0 other than those required to give access to the adjacent director area(s).

  3. Barring individual extensions (equipment located at the P.A.B.X.) - Selected extensions are barred access to the codes concerned by means of a strap in the extension line circuit, the exchange call being disconnected and N.U. tone returned from the P.A.B.X. when a prohibited code is dialled.

  4. Barring individual extensions (equipment located in the public exchange) - Barring is effected by the provision of a diode across the dial pulsing contacts of the extension instrument. A Yale-type lock may be provided on the telephone so that the barring may be made inoperative when required. This method of barring may also be used when the barred trunk equipment is located at the P.A.B.X. if the use of a Yale lock is required. The barred trunk equipment, on recognising a prohibited code dialled by an optionally barred extension, makes further dialled digits ineffective and N.U. tone is eventually returned to the calling extension from the public exchange.

(h) Additional barred trunk codes
Equipment generally of the type described in (g) but with the facility of barring additional codes may be provided at the P.A.B.X. to meet the particular needs of a subscriber.

(j) Private metering
The following types of private metering are available:-

  1. Exchange line metering - A 'totals meter' (cyclometer type) per outgoing or bothway exchange line to register the cumulative total of call charge units on that exchange line.

  2. Switched trip metering - Trip meters are associated with exchange lines as required by means of a rotary switch to indicate the call charge for individual calls.

  3. Extension metering (combined type) - A 'combined totals and trip meter' provided at the extension. (The meter is supplied and installed by the P.O.). This facility is not available to satellite extensions.

  4. Extension metering (cyclometer type) - Cyclo'meter - type meters, which may be of the re-settable type if required, are fitted at a central point and associated with extension line circuits. The meters may be either permanently connected to particular line circuits or associated with line circuits as required by means of a jack-field. This facility is not available to satellite extensions.

  5. Cord circuit metering - 'Trip meters' (cyclo'meter type) directly associated with the position cord circuits.

(k) Dictation recording
Access to dictating recording equipment is obtained from a group selector level. Control of the recorders etc. is described in Specn. S 662.

(l) Night service switchboard
The switchboard which is of the cordless type has up to four connecting circuits each permanently associated with one exchange line, one 0 level circuit and one extension line circuit.

(m) Local emergency service
One or more circuits terminated on the P.A.B.X. switchboard and/or in a local reporting centre may be dialled by extensions for reporting an emergency. The circuits may be extension lines or accessible from a group selector level and may be arranged to present 'non-busy' conditions to further callers when in use. Under night service conditions a fire alarm system may be operated directly by an extension dialling the emergency code.

(n) Local emergency service display panel
A display panel which indicates the location of a calling extension may be associated with all types of emergency service.

(o) Optional access barring
The following types of access from all or selected extensions may be placed under the control of keys.

(i) Trunk access.

(ii) Direct exchange access.

(iii) Group selector level access.

(p) Blind operators' switchboard 
Tactile indicators, light sensitive probes or any other suitable indicating device approved by the P.O. may be employed.

(q) Supervisor's desk
The standard desk is equipped with the following circuits as required:-

(i) An extension line circuit for making and receiving calls via the P.A.B.X.

(ii) A bothway speech circuit to each switchboard.

(iii) A listening circuit to each manual switchboard.

(iv) Miscellaneous switching keys, e.g., line diversion keys.

Other types of supervisor's desk may be provided to meet special requirements.

(r) Line diversion
Lines may be diverted to alternative terminations by key operations, e.g. special night service arrangements and the diversion of exchange or inter-P.B.X. lines to data transmission equipment.

(s) Enquiry desk
A number of extension line circuits from selector levels and bothway circuits to the manual positions may be terminated on the enquiry desk. Hold and recall facilities should be available on the circuits to the manual positions.

(t) Staff location
An approved privately-owned and maintained staff location system may be connected to the P.A.B.X.

(u) Night watchman's reporting system
An approved privately-owned and maintained night watch ' man's reporting system may be associated with the P.A.B.X.

(v) Timing devices
Timing devices such as the Clock 44 may be provided at the switchboard to enable the operator to time manually connected calls. Tp.S.I. H24 III describes the conditions applying to the use of such devices.

(w) Automatic call distribution
These systems distribute relatively large amounts of incoming traffic over a number of answering desks. The calls being presented as and when the desks become available. Call queuing facilities may be associated with these systems.

(x) Intrusion by the operator on engaged inter-P.B.X. lines
The operator is able to intrude on circuits which are engaged on automatically connected calls.

(y) Key and lamp units
Key and lamp units other than those normally provided by the P.O. may be provided for use in conjunction with call distribution systems or to meet special demands for lamp signalling and key switching to a number of lines etc., at the answering point.

(z) Key calling
Key calling enables selected extensions, usually referred to as 'master stations' to make calls to a number of specified extensions by operation of an appropriate key, thus avoiding the necessity of dialling that extension number.

Two basic types of key calling equipment are in use. The first is integrated with the P.A.B.X. equipment and a call from the master station to a specified extension is routed over the normal P.A.B.X. line wiring of that extension. Thus a specified extension while engaged on a master station call tests 'busy' as far as normal P.A.B.X. calls are concerned.

The second type is a separate key calling system employing special wiring between the master stations and the extensions.

Conference and Loudspeaking facilities may be associated with key calling stations of both types if required.

(aa) Conference
A conference call may be set up between up to ten extensions or private circuits, or one exchange line and five extensions, or one exchange line and seven extensions when a conference amplifier is also provided. A larger number of circuits may be connected if interconnected amplifiers up to a maximum of four are provided. The setting-up of the conference is controlled by the switchboard operator using cords and jacks.

(ab) Howler
Howler facilities may be provided on the switchboard enquiry desk, or on the M.D.F.

(ac) Keysender
A keysender may be provided on the manual positions. The normal capacity is for ten digits but when requested by the subscriber this may be increased up to 20 digits.

(ad) Ancillary extension appearance
Ancillary calling appearance in the manual positions may be provided.

10. Non-standard facilities
Certain facilities with special features requiring examination by P.O. headquarters (e.g. the use of recorded announcements) and all facilities which have not been previously approved are classified as non-standard. Technical and operational approval is required before these facilities may be provided. More detailed information is contained in Specification S 945.


11. Automatic extension line circuits and linefinders SA 8151
The automatic extension line circuits are arranged in groups of 50 to form linefinder groups and each group is served by a maximum of ten linefinders. 50-point P.O. Type 2 uniselectors are used as linefinders. Extension line circuits 30-34 in each linefinder group can be modified for manual extension working (with, or without access from the final-selector multiple) if required. Each linefinder is directly connected to a 1st group selector and the relays for controlling the linefinder start etc., are accommodated in the 1st group selector.

12. 1st group selectors SA 8152 (2000-type) and SA 8211 (4000-type)
The selectors serving a particular linefinder group are mounted on one shelf and the start circuits are arranged in the form of a series chain. Access to the start chain is obtained via the bank contacts of a uniselector (see par. 36) and, by this means, the start signal is extended to successive selectors on the shelf. Thus, if a faulty selector is seized initially, subsequent stepping of the uniselector wipers will route the call to another selector serving that linefinder group. A further function of the uniselector is to distribute the traffic over the selectors and linefinders in each group.

1st group selectors are of the 100-outlet type. The outlets of the levels allocated for extension numbers are trunked either direct, or via 2nd selectors, to the final selectors. The first bank contact of each spare level (with the exception of level 1) is connected to the spare level circuit to provide N.U. tone. In the event of a mis-operation which steps the selector to level 1, the selector does not switch but returns to normal at which point dial tone is reconnected to the calling extension. Failure to dial within a period of 30-60 seconds will cause the selector to be released and the extension line circuit to be made P.G.

13. Final selectors SA 8153 and SA 8169 (2000- type)-SA 8213 and SA 8214 (4000-type)
These selectors are used at main P.A.B.X,s and at register-controlled satellite installations where the number of extensions provided does not exceed 100, and a separate group of final selectors without the trunk offering facility is provided for extension-to-extension traffic. Selectors SA 8153 and SA 8213 are of the 100-outlet regular type. Selectors SA 8169 and SA 8214 are similar but with automatic hunting on groups of 2-10 extension lines.

The selectors may be arranged by means of strap ' ping on the shelf jack U-points to provide either first party or calling party release facilities. First party release is used at main installations and calling party release at satellites.

In addition to the normal tones, N.U. tone is returned from the selector if a P.G. or an equipped but non-allocated extension number is dialled.

14. Final selectors SA 8172 and SA 8185 (2000-type)-SA 8225 and SA 8226 (4000-type)
These selectors are used at satellite P.A.B.X.s. Selectors SA 8172 and SA 8225 are of the 100-line regular type while SA 8185 and SA 8226 have group hunting facilities. Calling party release is provided on these selectors. An operator is able to intrude on an engaged connexion by operating the cord circuit ring key and when the called extension clears under these conditions, ringing is automatically applied to the extension line. Provision is also made for the operator call-in facility on calls connected by the P.A.B.X. operator. The circuits are otherwise similar to those used at main installations.

15. Lamp lighting circuit SA 8157
These circuits are trunked from level 0 of 1st group selectors if individual extension line lamps arc provided. When a calling extension dials 0, ring tone is returned and a condition is connected to the 4th wiper of the 1st group selector to cause the lamp of the calling extension on the manual switchboard to glow. The holding circuit applied to the 1st group selector is designed so that, when the operator inserts a cord-circuit plug in the jack of the calling extension, the 1st group selector and lamp lighting circuit are released. Two circuits are accommodated in each relay-set.

16. 0-level circuit SA 8168
Individual extension line lamps are not provided on the manual switchboards of large installations (see par. 48), and an alternative method is employed for signalling the operator when 0 is dialled. A common group of 0-level circuits, each with an individual lamp and jack on the manual switchboard, is provided. When an extension dials 0, ring tone is returned by the 0-level circuit and the associated lamp glows on the manual switchboard. The operator answers the call by inserting a cord-circuit plug in the appropriate answering jack. These circuits are used as 'booking' circuits, the calls being completed via the extension multiple jacks, thereby freeing the 0-level circuit for further calls. Two circuits are accommodated in each relay-set.

17. Exchange line circuit SA 8158
Bothway and unidirectional exchange lines are terminated on the one type of exchange line circuit at the P.A.B.X. end. At installations with direct access to the public exchange, the relay-sets which require outgoing access from the automatic equipment are cabled to the bank contacts of level 9 of the 1st group selector multiple. the number of exchange line circuits is more than ten, it is usual for the circuits to be divided into groups of bothway, incoming only, manual outgoing only, and auto. outgoing only. For the 'outgoing auto. access only' exchange line circuits, a lamp and jack is provided on the manual switchboard to enable the operator to be called into circuit for assistance on directly dialled exchange calls.

Earth calling conditions from the P.A.B.X. to the public exchange are standard but at some early installations loop calling conditions were provided. Details of the public exchange arrangements are included in General, M 3902.

18. Exchange line adapter circuit SA 8154
This circuit is inserted on the line side of the normal exchange line relay-set when the public exchange is of the C.B.S. type. A transmission feed is provided in the relay-set for the extension instrument and the public exchange ring signal is repeated into the P.A.B.X. Two circuits are accommodated in each relay-set.

These adapter circuits are also used in conjunction with the normal exchange line circuits for the provision of trunk subscriber circuits, when, for transmission reasons, it is necessary to provide feed current at the P.A.B.X. end.

19. Exchange line adapter circuit SA 8155
The function of this circuit is similar to the SA 8154 adapter circuit and the circuit is provided when the public exchange lines are connected to a magneto exchange. An automatic 'ring off' signal is passed to the public exchange on the completion of the call.

20. Enquiry circuits SA 8159 and SA 8171
Two types of enquiry circuit are available. Both circuits are identical in circuit design but, in the SA 8159 the components are strip-mounted, while for SA 8171 a plug-in relay-set is used.

Enquiry circuits are associated with the exchange line circuits to provide 'enquiry' and 'operator recall' facilities on exchange connexions. Outlets from the exchange line circuits are multipled on the banks of the enquiry circuit uniselectors. 25-pt or 50-pt. uniselectors are employed, depending on the size of the installation concerned. Each enquiry circuit is directly connected to an enquiry selector which, in the setting-up of an enquiry call, functions as a 1st selector.

21. 2nd group, incoming or enquiry selector SA 8160 (2000-type) and SA 8212 (4000-type)
The same design of selector is used for all three purposes. The incoming selector may be required when dialling-in inter-P.B.X. lines are provided. Depending on the function of the selector, access to particular levels is barred by means of the normal-post springs in the selector, e.g. level 0 is barred on an enquiry selector and level 9 is barred on incoming 1st selectors. N.U. tone is returned by the selector if a barred level is dialled.

Incoming 1st or enquiry selectors may be mounted in unequipped positions on the 1st group selector shelves if space is available.

22. Inter-P.B.X. line circuit and private circuit relay-sets
The various types of inter-P.B.X. line and private circuit terminations used at a main P.A.B.X. 3 are given in Table 1. Circuit SA 8190 may be used at all satellite P.A.B.X.s. Other circuits providing dialling facilities may be used at non-register satellite installations but not at register-controlled satellite installations.

The type of termination employed for a particular circuit depends upon the characteristics of the line concerned and the method of working requested by the subscriber, e.g. dialling-in or manual-manual.

On dialling-in circuits SA 8161, SA 8163 and SA 8215, the dialled call is routed via an incoming 1st selector or, alternatively, via the normal linefinder and 1st group selector. A directly connected selector is always employed with circuit SA 8240. The linefinder and 1st group selector method may be employed when the traffic over the inter-P.B.X. line is comparatively light. When inter-P.B.X. connexions are made via the linefinder banks the associated line relays are not required and special wiring arrangements are made to the wiring of extension line circuits 30 to 34 in each linefinder group 50 that if required the relays may be readily disconnected. If the incoming 1st group selector method of working is employed, then these selectors are provided on the basis of one per inter' P.B.X. line circuit.

A general idea of the uses of the various circuits may be obtained from Table 1. Where two diagram numbers are quoted for the same class of circuit, i.e. SA 8143 and SA 8191, SA 8109 and SA 8190, both SA 8191 and SA 8190 refer to later circuits which will be installed for new work.

The use of circuits SA 8109 or SA 8190 may not be apparent from the table but they are used primarily for terminating lightly loaded automatic private circuits, access to which is obtained by dialling a final selector number.

In all instances direct manual switchboard access is provided, individual calling lamps and jacks being provided for each private circuit or inter- P.B.X. line.

23. Manual extension circuit SA 8164
Manual extensions, with access from the manual switchboard only, may be provided by modifying certain automatic extension line circuits (see par. 11). If a large number of manual extension circuits are required and the automatic extension line circuits are needed for normal use, the additional manual extension circuits may be provided by using manual extension circuit SA 8164. Five circuits are accommodated in each relay-set.


Diagram No. Type of circuit Method of working Access at
P.A. B. X.
Line signalling Limiting line conditions Type distant end equipment
*SA 8161 Inter-P.B.X. extn. or inter-P.B.X. private cct. B/W auto.- auto. From manual and auto. (selr. level) Loop dialling inwards and outwards 1200 ohms Note 1 P.A.B.X.
*SA 8162 Inter-P.B.X. extn. or inter-P.B.X. private cct. Private cct. B/W manual-manual B/W manual-manual From manual and auto. (seir. level) From manual and auto. (selr. level) A-wire earth or genr. outwards with A-wire earth inwards Genr. out-wards with loop inwards 4200 ohms 2100 ohms P.A.B.X. or P.M.B.X. Telephone
*SA 8163 Inter-P.B.X. extn. or inter-P.B.X. private cct. Manual out and auto. in From manual and auto. (selr. level) A-wire earth outwards with earth dialling inwards 2000 ohms Note 1 P.M.B.X.
*SA 8143 *SA 8191 Inter-P.B.X. private cct. or private cct. B/W manual-manual Manual only B/W genr. or balanced battery Note 2 P.A.B.X. P.M.B.X. or Telephone
Diagrams in contractor's series Inter-P.B.X. private cct. or private cct. B/W manual-manual From manual and auto. (selr. level) B/W automatic generator Note 2 P.A.B.X.
SA 8144 Private cct. B/W manual-manual Manual only Genr. out-wards with loop inwards 1000 ohms (inclusive of instrument) Telephone
SA 8109
Inter-P.B.X. extn. or inter-P.B.X.



Private cct.

B/W manual-manual or manual out and auto. in


B/W manual-manual or manual out and auto. in

From manual and auto.(final selr. number)


From manual and auto. (final selr. number)

A-wire earth or genr. outwards with A-wire earth or earth dialling inwards.

Generator out-wards with loop or loop-dialling inwards


2400 ohms Note 1


3000 ohms with aux. apparatus (e.g. U.A.A., 18) distant end 1000 ohms to C.B. telephone




*SA8193 Private cct. B/W manual-manual From manual and auto. (selr. level) Automatic balanced battery 6000 ohms P.A.B.X. or P.M.B.X.
SA 8200 Automatic private cct. Manual or auto. out arid auto. in From manual and auto. (final selr. number) Generator out-wards with loop-dialling inwards 600 ohms (1000 ohms if used with SA 8104) Telephone
*SA 8215 Inter-P.B.X. extn. or inter-P.B.X. private circuit B/W auto.-auto, From manual and auto. (selr. level) S.C.D.C. 8200 ohms and 76 500 SF/ohms P.A.B.X. or P.M.B.X.
*SA 8240 and SA 8299  Inter-P.B.X. extn. or inter-P.B.X. private circuit  B/W auto.-auto. or manual out and auto. in  From manual and auto. (selr. level)  1V.F.  Line loss 17 dB at 2280 Hz  P.A.B.X. or P.M.B.X. 
*SA 8178 B/W circuit to satellite Manual or auto. out and auto. in From manual and auto. (selr. level) Loop-dialling inwards and outwards Earth calling towards manual board 900 ohms Note 1 P.A.B.X. satellite
*SA 8174 O/G circuit to satellite Manual or auto. out From manual and auto. (selr. level) Loop-dialling outwards 1100 ohms Note 1 P.A.B.X. satellite
SA 8173 I/C circuit from satellite  Auto. in - Loop-dialling and earth calling to manual board inwards 1400 ohms Note 1 P.A.B.X. satellite
SA 8177 Circuit to main P.A.B.X. Auto. out From auto. (selr. level) Loop-dialling and earth calling to manual board inwards 14000 ohms Main P.A.B.X.

* Free-line signalling is available on these circuits.

NOTE 1:- The limits quoted may be varied when the circuits form part of a network with tandem dialling facilities. Details of the modified limits will be quoted in C 3005. Until this instruction is available queries should be referred to the Telecomms HQ (M2. 1.1.).

NOTE 2:- No limit if signal conversion and/or repeating equipment is provided. For other circuits see Diagram N 705.

24. Long extension auxiliary circuit SA 8104
Where the loop resistance of the automatic extension line exceeds 600 ohms, it is necessary to associate a long extension auxiliary circuit with the extension line circuit in order to ensure satisfactory operation of the line relay. Either three or six circuits may be accommodated in each relay-set.

25. 2nd group selector with one and two digit selection SA 8254 (2000-type) and SA 8239 (4000- type)
These selectors which are of the 100-outlet type may have automatic rotary hunting or pulse controlled rotary stepping with automatic hunting over a number of circuits. They provide access to any combination of single outlets and groups of from two to ten outlets and are used to provide access to inter' P.B.X. lines when a number of small routes exist.

26. Satellite alarm extension circuits
An alarm extension relay-set for extending prompt alarms to the main P.A.B.X. is associated with the last choice circuit incoming to the satellite P.A.B.X. When bothway lines are used circuit SA 8188 is provided at the main and circuit SA 8189 at the satellite P.A. B. X. When unidirectional lines are employed circuit SA 8176 is provided at the main and circuit SA 8175 at the satellite P.A.B.X. When an alarm condition arises at the satellite P.A.B.X., an earth condition is applied to the B-wire of the appropriate main satellite junction and the associated alarm lamp on the manual switchboard glows. Operation of the receiving attention key transfers the alarm to a receiving attention lamp and restores tile unction to normal service. When the alarm condition at the satellite is removed the alarm extension circuits automatically return to normal.

27. Satellite night service switching circuit SA 8186
This circuit enables an operator to switch selected extensions at a satellite installation to main to satellite circuits. These circuits are connected manually at the switchboard to exchange lines to provide direct exchange night service. Access to the circuit is obtained from a number in the satellite final selector multiple. When the circuit is seized N.U. tone is returned; the operator carries out the intrusion operation and warn tone is returned when the switching is completed. To restore the circuit an operator dials the same number; warn tone is returned when the circuit is seized, and the operator then intrudes and N.U. tone is returned when switching is completed.

28. Emergency circuit SA 8219
This circuit is used to terminate a local emergency service on the manual positions. The circuit is associated with a final selector number and when this number is dialled a relay is operated by the earth which is applied to the H-wire. The relay causes the switchboard calling lamps to glow and operates a distinctive audible alarm. The lamps and alarm are disconnected when the operator plugs into the answering jack.

29. Satellite register circuit SA 8245
When a satellite extension originates a call a register circuit is seized via the satellite finder circuit SA 8244. 'fhe register circuit returns dial tone to the calling extension. Storage for up to four digits is provided and the register circuit, by controlling the action of time satellite finder circuit, routes the call to a local selector exchange line or main satellite junction as required. Discrimination takes place after one or two digits have been received and up to four stored digits may he pulsed out. The action of the register circuit on the various types of call is as follows:-

(a) Calls to local extensions
After receipt of one or two digits the register causes the satellite finder circuit to switch to a local selector and releases. This action takes place within an inter-digital pause, enabling the extension to dial the subsequent digits without pausing.

(b) Calls to direct access exchange lines
On receipt of the digit 9, the register causes the satellite finder circuit to seize an exchange line. The register releases and dial tone is returned to the extension from the public exchange.

(c) Calls to the main manual switchboard
On receipt of the digit 0 the register circuit causes the satellite finder circuit to seize a main satellite junction. A discriminating relay is operated in the junction circuit to give manual calling conditions. The register releases.

(d) Calls to or via the main P.A.B.X. (excluding manual switchboard calls)
After sufficient digits have been received to enable discrimination to take place the register circuit causes the satellite finder circuit to seize a main satellite junction. In the case of extension-to-extension calls the register accepts the remaining digits dialled by the extension and pulses out all stored digits. The register circuit then releases. On calls to inter-P.B.X. circuits the register releases after pulsing out the required routing digits and dialling tone is returned from the inter-P.B.X. circuits.

30. Satellite finder circuit SA 8244
At register-controlled satellite P.A.B.X.s extension line circuits are connected in groups of 50 to the banks of the finder circuits. Local selectors, direct access exchange lines and inter-P.B.X. lines to the main P.A.B.X. are arranged in groups on the outlets of the circuit finder. Each finder circuit includes two 50-point P.O. Type 2 uniselectors, one used as an extension line finder and the other as a circuit finder. The finder circuit routes a call under the control of a register circuit SA 8245 (par. 29 refers). When a finder circuit is seized the associated circuit finder immediately hunts for a local selector. This enables the register circuit to be released after discrimination on calls to local extensions and subsequent digits to pulse the local selectors directly. On other types of call the local selector is released and the appropriate type of circuit seized when discrimination takes place.

31. Congestion circuit SA 8246
A congestion circuit is associated with each group of circuits connected to the outlets of a satellite circuit finder and returns engaged tone when all the circuits in the group are occupied.

32. Auxiliary selector level access and dial tone circuit SA 8288
This circuit is connected to selector level outlets used at main P.A.B.X.s to provide access from the extensions at a register-controlled satellite installation to inter-P.B.X. circuits employing S.S.A.C. 13. Dial tone is returned to the calling extension when the inter-P.B.X. circuit is seized.


33. General
At all new installations ringing and tone supplies are generated by ringing machines. Standard provision is one machine at installations with up to 400 extensions and two machines, with automatic change-over facilities, above this figure. At earlier installations vibrator ringing tone supplies were provided.

34. Ringing machine
Dynamotors 44 are provided at all new installations the circuits used being SA 8207 (ringer control circuit) SA 8208 (ringer common services) and SA 8209 (ringer rack circuit). At some earlier installations Dynamotors 23 were installed, the equivalent circuits being SA 8197, SA 8198 and SA 8199. At installations with 1200 or more extensions ringing and tone distribution circuit SA 8182 is provided.

35. Vibrator ringing
The general arrangements for vibrator ringing are shown on Dgm. SA 8165. Ringing and tone circuit SA 8106 and ringing pulse circuit SA 8107 are provided on the basis of one each per 200 automatic extension lines. Each ringing and tone circuit has a ringing pulse circuit associated with it which, besides generating the necessary pulses for interrupting the ringing and tones, also supplies the 'A' and 'Z' pulses required on some selectors and relay-sets and it also controls the linefinder start distribution.

The pulses are generated by means of a uniselector contained in the pulse circuit relay-set and four arcs of this uniselector are used for linefinder start distribution. Since each arc will serve one linefinder group, it follows that four linefinder groups, i.e. 200 extensions, can be served by one ringing pulse circuit. The supply of tones, etc. (for the inter-switchboard line relay-sets, etc.) are distributed on a rack basis over the number of vibrator sets installed. The load is, therefore, equally apportioned to the number of ringing vibrator relay-sets. When vibrator ringing is used for the automatic equipment, a separate vibrator is employed for generating the ringing supply for the manual switchboard. This vibrator is included in the manual ring and alarm circuit SA 8156 which is provided on the basis of one per installation. The necessary relays for extending the automatic alarms to the manual suite are also included in this relay-set.

36. Linefinder start distribution
The arrangements for installations with vibrator ringing are described in par. 35. a ringing machine is used pulse distribution circuit (SA 8195) is provided on time basis of one per 200 extension lines, at installations with up to 1200 extensions. Above that figure one relay-set SA 8179 is used to generate the pulses for the whole installation, and this common supply is distributed to the required racks by means of relays.


37. General
The following urgent alarms are provided:-

(a) Mains fail.

(b) Ring fail.

(c) Selector release.

(d) Fuse alarm.

(e) Manual ring fail (installations with vibrator ringing only).

(f) Manual fuse alarm.

The method of alarm indication depends upon the size of the installation. At main installations of less than 1200 extensions and at satellite installations the 'ring fail', 'selector release' and 'fuse' alarms are indicated on the particular rack and also signalled on the auto. alarm or satellite alarm lamp on the manual switchboard cable-turning section (C.T.S.). The 'mains fail' alarm and 'manual ring fail' alarm appear on the C.T.S. only, but the 'manual fuse' alarm is signalled on the manual position concerned. The relays concerned with the extension of the alarms from main installations to the C.T.S. are included in the manual ring and alarm relay-set SA 8156 where vibrator ringing is used and manual alarm circuit SA 8196 at other installations. Satellite alarm extension arrangements are described in par. 26. The various rack alarm circuits are included on the rack common services (Diagram SA 8170) for installations with vibrator ringing and Diagram SA 8210 for installations with machine ringing.

The alarm scheme adopted for main installations exceeding 1200 extension lines is shown on the rack common services (Diagram SA 8180) and follows main exchange practice; the equipment is divided into sections and sub-sections, and the alarms are only extended to the manual suite when the alarm extension key is operated.

A deferred alarm is provided for the indication of P.G. extensions. At installations where 'lamp per line' working is adopted, the P.G. indication is given on the C.T.S. so that the operator can, by means of the P.G. test key, locate the extension concerned. At main installations where 0-level working is employed and at satellite P.A.B.X.s, lamps are provided on the automatic equipment racks, to indicate in which line ' finder group the P.G. extension exists.


38. General
Equipment racks 7 ft. 6 in. to 7 ft. 9 in. high are generally used for installations of up to 1000-1200 extension lines. For larger installations, providing suitable accommodation exists, racks 8 ft. 6 in. high may be employed.

The racks described in this Instruction are typical only and reference should be made to the manufacturer's typical rack drawings for details of the exact number of selectors or relay-sets which may be accommodated on the racks supplied by that manufacturer. The following types of racks are normally provided:-

39. Line-and-final selector rack (7 ft. 6 in. high and 3 ft. 9 in. wide)
Fifty extension line circuits and ten linefinders are mounted on a composite shelf and are wired to form one linefinder group. Two such shelves, i.e. calling equipment for 100 extension lines, can be accommodated on each rack. The lower two shelf positions are used for this purpose. All points that require access for the purpose of inter-connexion are terminated on the shelf terminal strips.

The upper two shelves are used for mounting the final selectors. These selectors provide access to the 100 extensions, with calling equipments mounted on the lower shelves of the rack. A tie cable is provided between the two shelf multiples, and each final selector number is wired to its corresponding extension line circuit number on either shelf A or B. Limited I.D.F. facilities are provided on the shelf terminal strips.

The typical rack shown will accommodate 16 final selectors. Ringing and tone and ringing pulse circuits of the type referred to in par. 35 are mounted on the line circuit shelves. These relay-sets are not installed at installations with machine ringing and at other installations are provided on the basis of one per 200 automatic extensions, the shelf position being equipped on alternate racks only.

40. Group selector rack (7 ft. 6 in. high and 4 ft. 6 in. wide.)
A typical group selector rack is shown in Fig. 5. Five shelves may be provided on each rack and these are used for mounting 1st group selectors, enquiry selectors, incoming selectors and/or 2nd group selectors depending on the requirements of the particular installations. One 1st group selector shelf is provided for 50 extension lines, and grading facilities are provided on the terminal strips mounted at the rear of the shelves.

41. Relay-set rack (7 ft. 6 in. high and 4 ft. 6 in. wide)
The number of racks provided at a particular installation depends on the total number of exchange line and inter-switchboard line relay-sets required. As far as possible, the shelf jack wiring and shelf spacing are arranged so that exchange-line or inter-switchboard line relay-sets, and lamp lighting or 0-level circuits, may be mounted in the same shelf positions, without alteration to the permanent wiring of the shelf.

The types of shelves provided depend upon the requirements of the installation.

42. Automatic equipment rack (7 ft. 9 in. high and 4 ft. 6 in. wide)
Up to the present this size of rack has been provided by one manufacturer only. Accommodation exists on each rack for 100 extension line circuits, 20 linefinders, 20 1st group selectors, and 18 final selectors. The relay-set rack used may be of similar dimensions, or a smaller version (2 ft. 9 in. wide) may be provided if the number of relay-sets required at ultimate date can be accommodated on it.

43. Composite rack (7 ft. 6 in. high and 4 ft. 6 in. wide)
Composite racks equipped with any combination of group selector, relay-set or ringing machine shelves may be provided. One manufacturer has also provided at satellite installations a rack with accommodation for one 50 extension linefinder group the associated 1st group selectors, 10 final selectors and 20 relay-sets.

44. Ringer rack (7 ft. 6 in. high and 2 ft. 6 in. wide)
This rack accommodates one or two ringing machines, the associated control relay-sets and other miscellaneous apparatus.

45. Satellite register rack (7 ft. 6 in. or 7 ft. 9 in. high and 4 ft. 6 in. wide)
This type of rack accommodates the registers, finder circuits and circuit finders required to serve 100 extensions.

46. Equipment rack (8 ft. 6 in. high)
An economy in floor space can be achieved if racks 8 ft. 6 in. high are used. Use of these racks is normally restricted to large installations where sufficient ceiling height can be obtained. The capacities of equipment which may be accommodated on racks of this height are as follows :-

(a) Line circuit and linefinder rack - 200 extension line circuits and 40 linefinders.

(b) Group selector rack - 6 shelves (60 selectors).

(c) Final selector rack - 6 shelves (60 selectors).

(d) Relay-set rack - Shelves as required.

The number of final selectors per 100-line multiple may be 10, 15 or 20 selectors depending on the ultimate requirements.


47. General
For installations with an ultimate capacity not exceeding 1200 extensions, Sections, Switch, P.B.X., SA 7560 (or equivalent) with the appropriate cable-turning section are used. Either the standard mahogany or light-oak finished may be used, depending on the subscriber's requirements. A general description of this switchboard is given in B 1010. Fig. 7 shows a P.A.B.X. 3 manual suite using this type of section. Where the ultimate capacity is between 1200 and 2000 extensions a similar manual position with increased height to give additional multiple space is used. Above 2000 extensions the P.A.B.X. 3 equipment is mounted within the bodywork of a public exchange type sleeve-control position.

48. Face equipment
A four-panel multiple is adopted and the arrangement of jacks and lamps is as follows :-

(a) Exchange lines - From the pilot rail, the first eight spaces are reserved for exchange-line jacks and lamps, in strips of ten. Each lamp jack is mounted immediately above its respective multiple jack. More than one lamp appearance may be provided, in which case the 'primary lamp' is fed from the P.A.B.X. battery and the 'secondary' or 'ancillary' appearances are fed from a 6V a.c. supply. The primary appearance may be associated with any respective jack in the multiple, if distribution of the traffic over the positions during 'mains fail' conditions is required. Free-line signalling may be provided, in which case either an additional strip of 10 lamps is fitted immediately below the multiple jack or a 20-way lamp jack, which accommodates both the free-line signalling and calling lamps for ten circuits, is fitted immediately above the multiple jack.

(b) Inter-P.B.X. line and private circuits - Lamps and jacks for these circuits are fitted in the exchange-line multiple space and similar arrangements are made. The circuits on which free-line signalling may be provided are indicated in Table 1. For inter-P.B.X. circuits on which dialling from the manual switch ' board is required, an additional jack strip is required.

(c) Extension line circuits - The multiple of the extension line circuit is arranged in one of two methods depending on whether the ultimate number is 800 lines or more.

(i) Under 800 extensions - The extension jacks and lamps are in strips of 20. Sufficient space is available per panel for the provision of 20 strips and thus 800 lamped circuits can be accommodated on four panels. Only one calling lamp per extension is permitted but the extension-line jacks are multiplied on every fourth panel. The calling lamp is always mounted immediately above the respective line jack. This arrangement is generally referred to as 'lamp per line' working.

(ii) Over 800 extensions - For installations with more than 800 lines the jack space available in the extension multiple is not sufficient to permit the provision of individual line lamps per extension, and 0-level circuits are used for obtaining manual switchboard assistance.

Four jack spaces per panel may be used for 0-level circuit jacks and lamps and, in the remaining spaces, extension-line jacks (in strips of 20) for 300 extensions can be provided. Therefore, 1200 extension lines can be accommodated on four panels of the standard switchboard. The arrangement of the 0-level circuit lamps and jacks is the same as for exchange-line circuits, except that strips of 20 may be used. A similar type layout is adopted when large switchboards are used to provide additional multiple space (par. 47 refers).

The number of jack strips mentioned in the preceding paragraphs may be adjusted to meet particular requirements.

49. Position equipment
(a) Cord circuits - The cord circuit diagram is Diagram SA 8135 and the relays etc., for three circuits are mounted in one relay-set. The Section, Switch, P.B.X., SA 7560 will accommodate a maximum of five relay-sets, i.e. 15 cord circuits per position. The cord circuit functions as a bridge-control circuit for an extension-to-extension call, but becomes a through circuit on extension-to-exchange calls, the necessary discrimination being made on the sleeve of the cord circuit.

An additional pair of dial/trunk offering cords and associated keys are provided per position.

(b) Position pilot lamp - A position pilot lamp is not normally provided on P.A.B.X. 3 switchboards.

50. Conference circuits SA 8216 and SA 8217
Conference connexions are made on the switchboard by means of special jacks and the normal cord circuits. Circuit SA 8216 provides terminations for one exchange line or inter-P.B.X. line and two extensions and SA 8217 for three extensions. Diagram SA 8218 shows the complete conference arrangements. If the size of the conference arrangement exceeds ten extensions or one exchange or inter-P.B.X. line and five extensions one or more conference amplifiers must be provided. Each amplifier provides capacity for one exchange or inter-P.B.X. line and seven extensions and up to four amplifiers may be interconnected in accordance with Diagram SA 7010.

51. Metering circuits SA 8227 and SA 8228
Exchange and switched metering arrangements are shown on Diagram SA 8227. For exchange metering a meter and metering unit is permanently connected to each exchange line with metering facilities. For switched metering the usual arrangement is to connect the exchange lines to meters and metering units by use of a rotary switch but, alternatively, the meters only may be switched, the metering units being permanently connected to the exchange lines. This arrangement is only likely to be economic when used in conjunction with the exchange metering circuit.

52. Cable-turning section (CTS) equipment
For installations under 1200 lines, a Lamp, Alarm-indicating, No. 5 (Diagram SA 8145) is mounted in the

C.T.S. At larger installations suitable arrangements are made to mount the necessary alarm lamps and associated keys in the C.T.S. The audible alarm bell or buzzer may be mounted in the C.T.S.

Satellite alarm lamps are provided in the C.T.S. when required.

53. Angle sections
30' angle sections are supplied by the manufacturers, when required. Each section is equipped with one multiple panel.


54. General
The desk equipment may be installed in either special floor standing consoles or in key and lamp units designed to be mounted on an ordinary flat topped table, as required.

55. Supervisor's desk
The circuits employed are as follows

(a) Position circuit SA 8201 - This circuit provides for the connexion of one or more of the line circuits to the supervisor's telephone.

(b) Extension line circuit SA 8203 - The termination for one outgoing only or bothway extension line to the P.A.B.X. is provided for. Incoming calls cause the calling lamp associated with the circuit to glow. The call is answered by operation of the speak key. Outgoing calls are made by operating the speak key and dialling the required number. Incoming calls via the manual switchboard and outgoing calls may be held independently of the speak key by operation of the hold key.

(c) Bothway circuit to manual board SA 8204 - This circuit provides a means whereby a P.A.B.X. operator can communicate with the supervisor, to obtain information or assistance with a call. The supervisor uses this circuit to speak to an individual position. Access from the manual positions is via a jack. The supervisor gains access by the operation of a speak key. Calls to and from the P.A.B.X. or public exchange may be extended to the supervisor over this circuit and calls may be held as on the extension line circuit SA 8203.

(d) Instruction circuit SA 8205 - This circuit enables the supervisor to pass instructions simultaneously to all the manual positions. Unidirectional calling, from the supervisor to the operators only, is provided.

(e) Listening-in circuit SA 8206 - This circuit enables the supervisor to monitor any one of the manual positions. No calling or supervisory signals are given at either end.

(f) Buzzer and call bell circuit SA 8202 - This circuit includes the position buzzer and buzzer cut-off key and makes provision for call bells fitted in convenient positions to be rung by operation of a non-locking key.

56. Enquiry desk
The desk is equipped with the following circuits as required:-

(a) Position circuit SA 8223 - This circuit provides for the connexion of one or more key ended circuits to the enquiry operator's headset.

(b) Extension line circuit SA 8203 - As described in par. 55 (b).

(c) Bothway circuit to manual board SA 8204 - As described in par. 55 (c).

(d) Selector level access circuit SA 8224 - This is a unidirectional circuit incoming to the enquiry desk. It provides for the lighting of a calling lamp on the desk and returning ring tone to the caller. A hold key is provided.


57. General
The main 50V power supply is described in C 3001. A 6V a.c. supply is provided only when free-line signalling and/or ancillary lamps are required. The arrangements made are shown on Diagram SA 8184. When a 6V a.c. supply is available, it is also used for P.G. testing, but otherwise a 6V tap is taken from the P.A.B.X. battery for this purpose.


58. General
Either 'direct exchange' night service and/or a 'subsidiary night service switchboard' may be used for obtaining public exchange service during the period when the P.A.B.X. manual switchboard is not staffed (see also par. 9).

59. 'Direct exchange' night service
An additional manual switchboard jack is employed for extensions requiring direct exchange night service. This jack is located above the extension multiple and one strip is provided for every ten extensions on which the facility is required. The purpose of the jack is to disconnect the automatic extension line circuit from the extension line, when the cord circuit plug is inserted. Under these conditions, the incoming test battery is also disconnected from the final-selector multiple and N.U. tone is received if the extension number is dialled.

60. 'Subsidiary night service switchboard'
For this method, a night service switchboard SA 8166 is normally provided. The switchboard is usually located in the gatekeeper's office, or similar convenient point, and a maximum of four exchange line circuits can be terminated on it. The trunking elements under night service conditions are shown in Fig. 8. For simplicity only one circuit is shown.

The switchboard consists of the required number of key operated connecting circuits each of which enables an exchange line to be connected to a bothway-access circuit to the P.A.B.X. automatic equipment, or vice versa. At night, selected exchange line circuits are switched from the P.A.B.X. manual switchboard to the exchange line positions on the subsidiary board. The number of exchange line circuits extended is equal to the number of connecting circuits on the subsidiary switchboard.

The P.A.B.X. side of each connecting circuit is wired to a night service access circuit SA 8167 or SA 8192. Each night service access circuit has a connexion from level 0 of the 1st group selector multiple and is also permanently associated with an automatic ex ' tension line circuit. Thus, when an extension dials 0, the night operator can answer the call, or alternatively, the operator can dial calls to P.A.B.X. extensions via the access circuit, associated extension line circuit and the normal switch train. When in use on an incoming call to the P.A.B.X. the circuit is busied on the 0 level multiple.

Thus, incoming exchange calls answered at the night switchboard may be extended by dialling, to any P.A.B.X. extension, or alternatively, an extension dialling 0 may be answered at the switchboard and be connected to a public exchange line.

Extension of the selected exchange lines and the lines from the night access circuits to the subsidiary switchboard is controlled by the night service key. When this key is normal, i.e. during day service, the exchange lines are connected to the P.A.B.X. manual switchboard and the night service access relay-sets are strapped to function as normal lamp lighting or 0 level circuits, as described in pars. 15 and 16 depending on the type of working employed.

At large installations where night service access circuits are used, it may be necessary to provide an additional number of lamp lighting or 0 level circuits in order to deal with the manual switchboard traffic. If this is done, operation of the night service key will also busy the normal relay-sets on level 0 of the group selector multiple so that, at night, testing will occur on the night service access circuits only.

The standard night service switchboard circuit SA 8166 is only suitable for handling exchange line traffic. If night operation on inter-switchboard circuits is also required the Telecomms HQ (M2.1.1.) should be consulted for details of circuit modifications. Alternatively, standard cord-type P.M.B.X. switchboards may be employed.


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Last revised: April 27, 2023